In a spindle motor for rotating recording media of information recording and reproducing apparatus such as a hard disk drive, as means for supporting the load in the axial direction of rotor and suppressing deflection of rotor rotation, various thrust type dynamic pressure bearings are proposed, which are designed to generate dynamic pressure by herringbone grooves having middle flex part (for example, see Japanese Patent Registration No. 3155529).
Referring first to FIG. 14 and FIG. 15, the thrust type dynamic pressure bearing for generating dynamic pressure by herringbone grooves is briefly explained below. FIG. 14 (a) is a sectional view explaining the structure of a conventional thrust dynamic pressure bearing, FIG. 14 (b) is a sectional view explaining the structure of another conventional thrust dynamic pressure bearing, FIG. 14 (c) is a diagram showing a groove pattern shape of a bearing surface of a conventional thrust dynamic pressure bearing, and FIG. 15 is a graph showing the pressure distribution in the radial section of a herringbone groove of a conventional thrust dynamic pressure bearing.
FIGS. 14 (a) and (b) show the thrust dynamic pressure bearing having herringbone grooves in the prior art, and specifically FIG. 14 (a) shows bearing surface 21 at the face side of bearing rotary side member 10, and FIG. 14 (b) shows bearing surface 21 at the reverse side of bearing rotary side member 10. In the axial direction of rotation center axis 1, mutually opposing bearing surface 11 of bearing rotary side member 10 and bearing surface 21 of bearing fixed side member 20 are formed across a tiny gap filled with lubricating oil 50, a seal is provided on the outer side of the tiny gap for forming air-liquid interface 51 of lubricating oil and air, and plural herringbone grooves 30 for generating dynamic pressure shown in FIG. 14 (c) are formed on bearing surface 11 of bearing rotary side member 10 at a specified pitch.
When bearing rotary side member 10 rotates in the arrow direction in FIG. 14, dynamic pressure is induced in lubricating oil 50, and when the dynamic pressure reaches the maximum near middle flex part 31 of herringbone grooves 30, a pressure peak appears, and the pressure distribution in bearing radial section S–S′ declines toward the inside and outside in the radial direction from the peak of around middle flex part 31 as shown in FIG. 15. The thrust dynamic pressure bearing having herringbone grooves 30 supports the load in the axial direction of bearing rotary side member 10 in this pressure peak portion.
The thrust dynamic pressure bearing having herringbone grooves 30 has its pressure peak in middle flex part 31 of herringbone grooves 30 apart from rotation center axis 1 in the radial direction, and is hence higher in rotation rigidity as compared with a thrust dynamic pressure bearing having spiral grooves showing pressure peak in the center of the bearing. That is, the thrust dynamic pressure bearing having spiral grooves only supports the load in the axial direction, while the thrust dynamic pressure bearing having herringbone grooves not only supports the load in the axial direction, but also functions to suppress deflection of rotation of bearing rotary side member 10 owing to its high rotation rigidity. It is hence suited as thrust dynamic pressure bearing of thin type spindle motor difficult to assure a sufficient radial bearing length.
Usually, in the thrust dynamic pressure bearing having herringbone grooves 30, a centrifugal force due to rotation of fluid flowing in the so-called pump-out portion is applied to the inner side, and a strong force for attracting the fluid to the outer side works near the axial center of the gap of the thrust dynamic pressure bearing. Accordingly, when rotating at high speed, a negative pressure is likely to occur in the region of the inner side from the pump-out portion. In such negative pressure, bubbles are likely to be formed in the fluid, and these bubbles are believed to induce decline of bearing performance, in particular, performance decline of thrust dynamic pressure bearing. To suppress these bubbles, it has been proposed to suppress generation of negative pressure at the inner side and prevent occurrence of bubbles by dislocating middle flex part 31 of herringbone grooves 30 to the inner side, and reinforcing the pumping action in the pump-in portion at the outer side from the pump-out portion of the inner side (for example, see Japanese Laid-open Patent Publication No. 2001-173645).
If bubbles mix in lubricating oil 50 due to some reason, the bubbles move from a higher level to a lower level of pressure along the pressure gradient. As described above, the pressure distribution in the radial section of thrust dynamic pressure bearing has its peak near middle flex part 31 of herringbone grooves 30 as shown in FIG. 15 regardless of the position in the section, showing a mountain-like profile becoming lower toward the inside and outside in the radial direction. Accordingly, the mixing bubbles move, as shown in FIG. 15, after reaching the pressure peak of middle flex part, to the lower pressure parts, that is, the inner periphery of the bearing or the outer periphery of the bearing. The bubbles moving to the outer periphery of the bearing are discharged outside of the bearding from air-liquid interface 51, but bubbles moving to the inner periphery of the bearing are not discharged, and stay within the inner periphery of the bearing. Thus, when bubbles are present in the bearing, the bubbles rotate without synchronism with the rotation of the rotor, the bearing rigidity fluctuates during rotation, deflection that is not synchronous with rotation is generated, and stable rotary motion of the shaft is spoiled. Further, if bubbles are stagnant in the inner periphery of the bearing, when exposed to a reduced pressure environment or high temperature environment, the bubbles expand and force the lubricating oil outside of the bearing to lead to a lubricating oil leak, or the leaking lubricating oil stains the outer parts of the bearing. Hence, various methods have been proposed for removing bubbles generated or staying in the bearing parts (for example, see Japanese Laid-open Patent No. 2004-112874, Japanese Laid-open Patent No. 2004-132535, Japanese Laid-open Patent No. 2004-183768, etc.).
Examples of eliminating bubbles generated or staying in the bearing parts of a thrust dynamic pressure bearing are explained below.
FIG. 16 (a) is an example of a structure for removing bubbles, showing a groove pattern shape of a bearing surface of a thrust dynamic pressure bearing. In this example, the thrust dynamic pressure bearing having spiral grooves is shown, instead of the thrust dynamic pressure bearing having herringbone grooves explained above. In FIG. 16 (a), at the radial inner side of thrust bearing 150 having spiral grooves 150a, radiant grooves 153 are formed from the radial inner end of spiral grooves 150a for generating dynamic pressure provided at the upper end of housing 151 of thrust bearing 150, to the radial inner end of the end face of sleeve 152. Bubbles mixing in oil are agitated by these radiant grooves 153 during rotation, and cracked finely, and are discharged from the bearing gaps axial grooves 155 and communicating holes 154, and hence bubbles mixing in the oil can be discharged.
FIG. 16 (b) is another example of a structure for removing bubbles, showing another groove pattern shape of a bearing surface of a thrust dynamic pressure bearing. In this example, too, the thrust dynamic pressure bearing having spiral grooves is shown, instead of the thrust dynamic pressure bearing having herringbone grooves explained above. In FIG. 16 (b), two spiral grooves 150a, 150b of pump-in type are provided as dynamic pressure generating grooves for generating dynamic pressure provided at the upper end of sleeve 152 as thrust bearing 150. One spiral groove is an ordinary spiral groove 150a for generating dynamic pressure, and the other spiral groove 150b is at least one extended to the radial inner side, and is formed larger in width in the peripheral direction than the ordinary spiral groove 150a. The other spiral groove 150b is larger in width in the peripheral direction so as to collect bubbles, and is extended to the radial inner side so as to discharge bubbles.
However, in the thrust dynamic pressure bearing having herringbone grooves, the thrust dynamic pressure bearing of the structure for suppressing generation of negative pressure in the region of the inner side and preventing generation of bubbles by dislocating middle flex part 31 of herringbone grooves 30 in the inner side and reinforcing the pumping action of pump-in portion at the outer side of the pump-out portion at the inner side requires not only much time in design of setting the position of middle flex part 31 and the groove shape of the dynamic pressure generating groove, but also requires higher precision than ordinary herringbone grooves, and therefore a new problem of soaring material cost must be solved.
Various proposals for discharging bubbles generated or stagnant in the thrust dynamic pressure generating parts mostly relate to thrust pressure generating parts of the spiral type, and almost nothing discusses the thrust dynamic pressure generating parts of the herringbone type. In the case of thrust dynamic pressure generation parts, between the spiral type and herringbone type, the principle of generating dynamic pressure is basically the same, but the fluid flowing direction is different. That is, in the spiral type, the fluid flows only in one direction in the outer direction from either the outer side or the inner side of the periphery of the thrust dynamic pressure generating parts. In the thrust dynamic pressure generating parts of the herringbone type, the fluid flows from both the outer side and the inner side of the periphery toward the middle flex part. Accordingly, the proposals of forming radiant grooves separately, partly extending plural dynamic pressure generating grooves, or expanding the width dimension in the peripheral direction seem to be effective for thrust dynamic pressure generating parts of the spiral type, but they can hardly be applied in the thrust dynamic pressure generating parts of the herringbone type.
Further, for the purposes of forming radiant grooves separately in the thrust dynamic pressure generating parts of the spiral type, partly extending plural dynamic pressure generating grooves, or expanding the width dimension in the peripheral direction, an extra area of circular plane for forming thrust dynamic pressure generating parts is required, or the bearing parts are increased in size, which is difficult to use in small size applications. To realize these proposals without increasing the area of circular plane for forming thrust dynamic pressure generating parts, the area of the portion for forming thrust dynamic pressure generating parts of the spiral type must be reduced, but the generating dynamic pressure is decreased in this case, and the operation as thrust bearing is difficult, and serious quality troubles may occur, and these are important problems to be solved.